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Prof Patrick Schrauwen of Maastricht University in The Netherlands, who led the study, said the benefits of resveratrol for obese people were small but significant.

He said: "I think the positive thing is that they were very consistent, they cause a small difference on a lot of different parameters.

"Also, we only gave it to patients for 30 days and we do not know what would happen if we gave it for longer. Metabolic changes can take a while before they start to appear so it is quite possible the effects could be larger."

Dr Andrew Murray, of Cambridge University, who was not involved in the study, said it provided the first real evidence that resveratrol could have a significant effect in humans.

He said: "Although the effects are slight they could make all the difference to people with metabolic complications like obesity.

"What is very exciting about this is that there are many problems related to obesity and the onset of diabetes, and what this study seems to show is that it is not going to make people thin again but it could slow down all the problems associated with lifelong obesity."

The researchers gave daily injections of resveratrol to 11 obese male patients for a month and found it altered their metabolism in a similar, although weaker, way than extreme dieting or endurance training.

The treatment lowered systolic blood pressure by 5mmHg and reduced the amount of energy participants used by two to four per cent, indicating that their metabolism had slowed down.

Biologically speaking this is healthy because it means we require less energy and need to eat less, but the researchers admitted that burning fewer calories may not be desirable for obese patients.

Prof Schrauwen said: "In our society with so much obesity people want to have a high metabolism because it is easier to lose weight.

"But the lowering of your energy metabolism is actually a good thing because it means that you become more efficient."

Because the eleven patients tested in the study, published in the Cell Metabolism journal, were all obese men it is not clear whether slimmer people would benefit in the same way.

Prof Schrauwen said: "I can imagine it will work better in people who already have some disturbances in their metabolism – if your blood pressure and blood glucose are normal, then it might not be lowered."